1. Field of the Invention
This invention relates to aluminum alloys, and specifically to high tensile strength aluminum-silicon hypoeutectic and eutectic alloys suitable for high temperature applications such as heavy-duty pistons and other internal combustion applications. It relates particularly to a process for producing a cast article from these high tensile strength aluminum-silicon hypoeutectic and eutectic alloys.
2. Discussions of the Related Art
Aluminum-Silicon (Alxe2x80x94Si) casting alloys are the most versatile of all common foundry cast alloys in the production of pistons for automotive engines. Depending on the Si concentration in weight percent, the Alxe2x80x94Si alloy systems fall into three major categories: hypoeutectic ( less than 12 wt % Si), eutectic (12-13 wt % Si) and hypereutectic (14-25 wt % Si). However, commercial applications for hypereutectic alloys are relatively limited because they are among the most difficult Al alloys to cast and machine due to the high Si contents. When high Si content is alloyed into Al, it adds a large amount of heat capacity that must be removed from the alloy to solidify it during a casting operation. Significant variation in the sizes of the primary Si particles can be found between different regions of the cast article; resulting in a significant variation in the mechanical properties for the cast article. The primary crystals of Si must be refined in order to achieve hardness and good wear resistance. For these reasons, hypereutectic alloys are not very economical to produce because they have a broad solidification range that results in poor castability and requires a special foundry""s process to control the high heat of fusion and microstructure. Furthermore, expensive diamond toolings must be used to machine parts, such as pistons, that are made from hypereutectic Alxe2x80x94Si castings. On the other hand, the usage of hypoeutectic and eutectic alloys are very popular for the industry, because they are more economical to produce by casting, simpler to control the cast parameters, and easier to machine than hypereutectic. However, most of them are not suitable for high temperature applications, such as in the automotive field, for the reason that their mechanical properties, such as tensile strength, are not as high as desired in the temperature range of 500xc2x0 F.-700xc2x0 F. Current state-of-the-art hypoeutectic and eutectic alloys are intended for applications at temperatures of no higher than about 450xc2x0 F. Above this elevated service temperature, the major alloy strengthening phases such as the xcex8xe2x80x2 (Al2Cu) and Sxe2x80x2 (Al2CuMg) will precipitate rapidly, coarsen, or dissolve, and transform themselves into the more stable xcex8 (Al2Cu) and S (Al2CuMg) phases. This undesirable microstructure and phase transformation results in drastically reduced mechanical properties, more particularly the ultimate tensile strength and high cycle fatigue strengths, for hypoeutectic and eutectic Alxe2x80x94Si alloys.
One approach taken by the art is to use ceramic fibers or ceramic particulates to increase the strength of hypoeutectic and eutectic Alxe2x80x94Si alloys. This approach is known as the aluminum Metal Matrix Composites (MMC) technology. For example, R. Bowles has used ceramic fibers to improve tensile strength of a hypoeutectic 332.0 alloy, in a paper entitled, xe2x80x9cMetal Matrix Composites Aid Piston Manufacture,xe2x80x9d Manufacturing Engineering, May 1987. Moreover, A. Shakesheff has used ceramic particulate for reinforcing another type of hypoeutectic A359 alloy, as described in xe2x80x9cElevated Temperature Performance of Particulate Reinforced Aluminum Alloys,xe2x80x9d Materials Science Forum, Vol. 217-222, pp. 1133-1138 (1996). In a similar approach, cast aluminum MMC for pistons using eutectic alloy such as the 413.0 type, has been described by P. Rohatgi in a paper entitled, xe2x80x9cCast Aluminum Matrix Composites for Automotive Applications,xe2x80x9d Journal of Metals, April 1991.
Another approach taken by the art is the use of the Ceramic Matrix Composites (CMC) technology in the place of hypoeutectic and eutectic alloys. For example, W. Kowbel has described the use of non-metallic carbonxe2x80x94carbon composites for making pistons to operate at high temperatures in a paper entitled, xe2x80x9cApplication of Net-Shape Molded Carbonxe2x80x94Carbon Composites in IC Engines,xe2x80x9d Journal of Advanced Materials, July 1996. Unfortunately, the material and processing costs of these MMC and CMC technology approaches are substantially higher than those produced using conventional casting, and they cannot be considered for large usage in mass production, such as engine pistons.
A primary object of the present invention is to provide a process for making a cast article from an aluminum alloy, which cast article has improved mechanical properties at elevated temperatures.
According to the present invention, an aluminum alloy having the following composition, by weight percent, is first provided:
In this aluminum alloy the ratio of Si:Mg is 10-25, and the ratio of Cu:Mg is 4-15.
An article is cast from this composition, and the cast article is aged at a temperature within the range of 400xc2x0 F. to 500xc2x0 F. for a time period within the range of 4 to 16 hours.
In a preferred embodiment, after an article is cast from this alloy, the article is heat treated in a solutionizing step which dissolves unwanted precipitates and reduces any segregation present in the said alloy. After the solutionizing step, the article is quenched, and is then aged at elevated temperature for maximum strength.